1. Field of the Invention
This invention relates to linear motors, or actuators, and, more particularly, to providing such a device with linear bearings extending along each side of an electromagnetic structure and having an adjusted level of pre-load force.
2. Description of the Prior Art
U.S. Pat. No. 5,153,472 describes an actuator for use in positioning a probe. The actuator has a magnet, two C-shaped cores on opposite sides of the magnet, an electromagnetic coil, and an armature movably connected with the cores. The armature has a projecting face located over portions of both pole faces for each core and for the entire range of motion of the armature. The armature is primarily retained with the cores by magnetic attraction by the magnet. The actuator also has an armature position sensor including a light source producing a light beam which is divided to go to two photodetectors. The portions of the divided light beam pass through oppositely oriented triangular apertures moving with the armature, so that, with such movement, the output of one photodetector increases while the output of the other photodetector decreases. The difference between these outputs provides an indication of the position of the armature. A test probe is mounted to the armature, which can be pre-loaded and have dual springs to prevent scrubbing and increase contact force with an object being tested.
U.S. Pat. No. 5,180,99 describes an X-Y positioning apparatus, comprising a multi-bar linkage drive mechanism which may be used to position the actuator of U.S. Pat. No. 5,153,472. The drive mechanism has a base structure and a movable arm structure. The movable arm structure has four arms arranged in a general parallelogram profile with a center of gravity at a main shaft located at a connection of two of the arms. Two of the arms have energizeable electromagnetic coils and the base structure has permanent magnets for moving the arms. Analog and/or digital position sensors can be used. Dynamic balancing can be provided to produce reactionless ultra-high speed X-Y motion. A torque cancellation motor for cancellation of the Z-axis moments produced by these coils can be provided.
Thus, the actuator of U.S. Pat. No. 5,153,472 is used to provide a probing motion in the Z-direction to engage a circuit under test with a probe, with the actuator being mounted on the X-Y positioning mechanism of U.S. Pat. No. 5,180,955. However, this arrangement provides a maximum travel or stroke of only one millimeter. While this arrangement is satisfactory for circuits, such as glass ceramics, which are very flat, what is needed is a Z-direction actuator having a substantially greater stroke, so that printed circuit boards having a warpage of, for example, two millimeters can be tested, and so that the probe may be moved over certain components of populated circuit boards under test.
U.S. Pat. Nos. 5,626,276 and 5,775,567 describe ultrasonic welding apparatus for wirebonding, thus providing an example of another type of application in which a small linear motor having a relatively long stroke can be usefully employed. While the tubular piezoelectric actuator of U.S. Pat. No. 5,775,567 is moved in a vertical direction in and out of engagement with the workpiece by means of a solenoid, U.S. Pat. No. 5,626,276 shows an example of a linear motor being used for this purpose.
Many linear motors in the prior art use linear bearing arrangements including grooved tracks in which balls rotate, with a first pair of such tracks being attached to the moving carriage, and a second pair of such tracks being attached to a stationary framework. Grooves in each such pair of tracks face opposite directions, and a number of bearing balls roll in a space between opposing grooves attached to the moving carriage and to the stationary framework. While, in a way, such a bearing arrangement is similar to a conventional rotary ball bearing, it is much easier to retain close clearances among the grooved bearing races of a rotary ball bearing and the balls retained between the grooves, since such clearances are dependent on the diameters of the grooves and of the bearing balls, with such diameters being easily maintained by modern manufacturing processes. On the other hand, the bearings used in linear motors generally include at least one pair of bearing tracks which are formed as separate pieces, so that it is relatively difficult to control the spacing between the grooves. Thus, what is needed is a method to adjust the spacing between two of the tracks, so that such spacings can be minimized.
A Japanese application, Publication No. of 60-167669, describes a linear motor having a pair of sliders with outwardly-disposed grooves, attached to a sliding carriage, moving within a guiding channel with inwardly-disposed grooves. On each side of the sliders, a number of bearing balls roll between opposing grooves in a slider and in the guiding channel, moving within a plate-shaped retaining cage with holes for the balls. The sliders are held apart, so that the clearance between the grooves and the balls rolling therein is minimized, by means of a wedge-shaped member clamped in place between the sliders by a mounting screw engaging the sliding carriage.
While this ability to minimize clearances provides the linear motor of Japanese application, Publication No. of 60-167669, with an important advantage over other prior art linear motors, the placement of the wedge-shaped member between the sliders requires the placement of the magnetic structure and the coil away from the sliders, so that a torque is developed along with the force causing acceleration when the carriage is moved. This torque results in reaction forces acting on the balls and grooves in the linear bearing arrangements.
U.S. Pat. Nos. 4,864,170 and 5,229,669 describe linear motors having bearings in which grooved channels including bearing balls are aligned with the magnetic structure and coils extending between and/or around the grooved channels, so that the torque developed during acceleration by means of magnetic forces is minimized. However, these linear motors do not have a provision for adjusting the distance between a pair of grooved tracks to minimize clearances around the bearing balls.
U.S. Pat. No. 5,763,965 illustrates another method for eliminating clearances around bearing balls in a linear bearing. On one side of the motor windings and the magnetic structure, bearing balls operate in a pair of opposing grooves in tracks, while on the other side, bearing balls operate between an upward-facing groove and a downward-facing flat surface. With this configuration, the effect of gravity removes clearances around bearing balls in a precision X-Y table. However, in a linear motor used in a high-speed probing application, the use of a constant force mechanism, such as gravity or a spring, to hold the linear bearings together, is not believed to be practical because of the high shock loadings experienced during contact of the probe with the circuit being tested.
Thus, what is needed is a linear motor having a method for minimizing or eliminating the clearance around bearing balls in a linear bearing, which also has a configuration placing the magnetic structure and electrically driven coils generally in alignment with the tracks of the linear bearing, extending within and/or around the linear bearing.